The fidelity and dynamic response of fluid-filled catheter systems for direct measurement of lumbar cerebrospinal fluid pressure.

OBJECTIVE

The purpose of this study was to determine the fidelity of pressure signals transmitted through long, narrow (epidural) catheters inserted into the lumbar intrathecal space.

METHODS

Using a model of the spinal canal we tested three epidural catheters: 20-gauge Arrow, 20-gauge Abbott, 21-gauge Portex. We (1) determined the damping coefficient and natural frequency of the three catheters, (2) correlated the static pressures measured using the three catheters compared to the true pressure in the intrathecal space, and (3) compared the response time of the three catheters connected to transducers vs U-tube manometers.

RESULTS

The three catheters had high damping coefficients (alpha) (Arrow, 0.75; Abbott, 0.85; Portex, 1.10) and low natural frequencies (Arrow, 15.23 Hz; Abbott, 12.83 Hz; Portex, 9.09 Hz). The dynamic response characteristics of the catheter with the largest internal diameter (20-gauge Arrow) were adequate to reproduce pulsatile cerebrospinal fluid pressure reliably. Smaller catheters tracked the mean pressure, although oscillations were damped. Static pressure measurements from all three catheters showed good correlation with test pressures (r = 0.99; p < 0.001). Using the U-tube manometer, it required 170, 140, and 130 minutes for the Portex, Abbott, and Arrow catheters, respectively, to equilibrate with a test pressure of 30 cm H2O. The rate of rise in the U-tube manometer pressure was limited by the rate of fluid flow through the catheters.

CONCLUSIONS

We found that a catheter of at least 20 gauge connected to a transducer could record pressures in the cerebrospinal fluid compartment with a high degree of fidelity. The prolonged time to reach equilibrium made U-tube manometry unsuitable for clinical use.